Understanding GNSS Sensitivity

GNSS Sensitivity is defined as the lowest signal level at which a GNSS receiver is able to track and achieve a position fix on overhead satellites. If the receiver receives a signal that is lower than this level, it is not able to get a position fix. GNSS receivers usually amplify the signal received from the satellites using Low Noise Amplifiers. However, while an LNA increases signal power, it degrades Signal to noise ratio (SNR). Thus, as the RF power level of a GPS signal increases, SNR decreases, and eventually, the receiver is no longer able to track the satellite. Hence a certain minimum level of signal is required from the satellite to allow the receiver to calculate its position. Sensitivity is measured in dBm and is one of the most important parameters of a GNSS receiver.

There are three main types of sensitivities specified for most GNSS receivers – Acquisition Sensitivity, Tracking Sensitivity and Navigation Sensitivity.

What is Acquisition Sensitivity?

Acquisition sensitivity represents the minimum power level at which a GNSS receiver can achieve a position fix. This is the minimum level to successfully perform TTFF under cold start. Cold Start is the TTFF condition where the GNSS receiver has no information about the satellites above. It needs to conduct a full search of the sky to find the satellites in view and obtain the almanac and ephermal data. Acquisition sensitivity is usually around -140 to -150 dBm.

What is Tracking Sensitivity?

Tracking sensitivity is the minimum power level at which a receiver can receive and maintain a connection with a satellite or constellation of satellites. In terms of TTFF states, this would be the Warm/Hot Start state. Here the receiver has information on the satellites in view and it needs to connect and get a position fix. The tracking sensitivity is usually lower than the acquisition sensitivity (-150 to 165 dBm). 

The tracking sensitivity level is closely related to measurement errors due to error sources in the GNSS receiver’s PLL - Phase error, dynamic stress error, and thermal noise are the dominant sources of error. Minimizing these errors will enable the receiver to continue to track signals at much lower power levels.

What is Navigation Sensitivity?

Navigation sensitivity is the minimum power level at which a receiver can maintain a connection with the GNSS Satellites to provide an accurate location during navigation.